The present invention relates to lasers and particularly to tunable lasers which produce laser emission at one or more wavelengths over a preselected wavelength range. Tunable lasers are lasers which have the capability of emitting one or more wavelengths over a broad range of wavelengths. These lasers are continuously tunable over a preselected range of wavelengths and differ from non-tunable lasers which emit at certain fixed, discrete wavelengths. As a consequence of their continuous tunability, tunable lasers have proven to be extremely useful in a wide variety of applications. Two vital characteristics that determine the utility of a specific tunable laser for a given application are the wavelength range over which the laser is tunable and the bandwidth of the emitted line. The broadest wavelength range for laser operation is determined by the gain bandwidth of the gain medium itself. Generally speaking, the wavelength range over which a tunable laser can operate is limited for a particular resonator configuration to the wavelength range for which the optical gain exceeds the sum of all of the losses, including passive loss, excited state absorption and output coupling loss. For a given laser resonator configuration the tuning range will therefore be affected by the pump intensity, by the wavelength range of the coatings on all of the intracavity optical elements including reflective and anti-reflective coatings, and the absorption of the various optical elements contained within the laser resonator cavity through which the resonator light must pass. The bandwidth of a given line, on the other hand, is controlled by the insertion of various dispersive or linewidth narrowing elements placed within the laser resonator. Dispersive elements would include prisms or diffraction gratings while non-dispersive means of reducing the linewidth would include intracavity etalon plates or birefringent wave plates. If the loss exceeds the gain for any given wavelength the laser cannot operate. Therefore, in determining the types of intracavity elements which may be used one must not insert elements into the cavity whose losses would be high enough to prohibit lasing of the gain material for a given pump power density at the desired wavelength.
Generally speaking, lasers, and specifically tunable lasers, can be excited by either cw (continuous wave) means, or by pulsed means. For pulsed excitation the initial population inversion created by the pumping source is substantially higher than the steady state or cw population inversion. Therefore, the gains that can be realized by pulsed excitation are often much higher than those that can be achieved in cw or steady state excitation. As a consequence, elements with relatively high loss can be inserted into the cavity when using pulsed excitation without eliminating the laser operation of the cavity. However, for cw operation, the population inversion is clamped near threshold and elements inserted into the cavity must have low insertion loss compared to the output coupling. A basic tunable laser consists of several components, including a gain or active medium, two or more reflective surfaces to form a resonator therebetween and a means for tuning or adjusting the wavelength of the laser emission. If no tuning element is inserted into the cavity the laser is described as "free-running" or "untuned" and it will operate at a wavelength close to the wavelength for which the net gain is highest. The net gain is defined as the gain minus the sum of all losses.
In selecting tuning elements for a cw or steady state laser a selection is made among tuning elements with low insertion losses. As mentioned before, suitable tuning elements for cw lasers include prisms and birefringent filters, or etalons can be used to tune over narrow wavelength ranges in a cw laser. Because prisms are dispersive and birefringent filters are not, there are applications for which a prism is a more appropriate tuning element than a birefringent filter. One such application involves multifrequency operation of a tunable laser, as is described in co-pending U.S. patent application Ser. No. 07/970,328 U.S. Pat. No. 5,276,695 entitled "A Multifrequency Rapidly Sequenced or Simultaneously Tunable Laser" by Richard Scheps.
There are several sources of optical loss caused by an intracavity prism. One source is due to reflections at the entrance and exit faces of the prism. In addition, diffraction losses occur when the prism aperture is small compared to the beam diameter. Finally, losses occur when the beam undergoes scattering and absorption as it is transmitted through the prism material. The material from which the prism is manufactured should have high dispersion to provide relatively fine wavelength adjustment. The degree of dispersion of a prism material is characterized by the Abbe constant of the material. Thus, the criteria for selecting a prism or prisms to insert into a tunable laser cavity are that the insertion loss due to reflection, diffraction, scattering and absorption by the prism material be kept to a minimum and that the dispersion of the prism be as high as possible.
The highest Abbe constants are generally associated with optical materials that have high Verdet coefficients such as those used in Faraday rotator glasses. Unfortunately these materials generally tend to have high scattering losses and absorb strongly at wavelengths between 800 nm and 1 micron. It is difficult to find a prism material with high dispersion and low insertion loss and design a suitable prism for use in a tunable laser operating at wavelengths from 700 nm to 1 micron. Furthermore, the use of a discrete tuning element in the cavity adds to the resonator passive losses and reduces the net gain.
Thus in accordance with this inventive concept a need has been recognized for an operational tunable laser which can be tuned without requiring a separate, discrete tuning element inside the laser cavity to provide the advantages of a low threshold and high operating efficiency tuning of the laser output wavelength.